The present invention concerns a method for the production of a porous ceramic body, especially a filter membrane and a porous ceramic body and its use in a filter, especially in a so-called cross-flow filter.
In a wide range of industrial areas, such as chemical engineering, food technology etc, liquid material streams (fluids) have to be filtered. To an extent depending on the size of the materials to be filtered out, a distinction is made between microfiltration (MF), ultrafiltration (UF) and nanofiltration (NF). The smaller the grains to be filtered out, the higher the demands imposed on the filter technology and the filter membranes used thereby.
For MF, UF and NF filter applications, so-called cross-flow filtration is known from the prior art in which the material stream to be filtered flows parallel to the filter surface. In the pressure-driven separating process of cross-flow filtration, the material stream to be filtered out is split by the filter membrane into two outlets streams, namely a purified stream, the so-called filtrate or permeate, and a second stream, the so-called residue stream or retentate.
By the agency of the perpendicular diversion of filtrate or permeate to the feed stream or filter membrane, two speeds can be distinguished in cross-flow filtration, more precisely the filtrate speed perpendicular to the filter surface and the cross-flow speed parallel to the filter surface. In cross-flow filtration, optimization of the ratio of filtrate speed to cross-flow speed is especially important. The cross-flow speed, which is the mean value of the material stream at the inlet and outlet at the filter, ranges in known industrial applications from 1 to 10 m per second, with a TMP (see below) of 2.5 bar producing in the test on water a flow of approximately 500 to 2500 liters per m2 bar per hour for MF and UF membranes having pore sizes ranging from 0.1 to 1 A 0.5 μm. To an extent depending on the application as filter, a flow of 50 to 500 l per m2 bar per hour usually becomes established.
An increase in the cross-flow speed generally leads to a rise in flow on account of a higher shear rate. This promotes more efficient removal of the grains from the material stream because a dynamic layer forms on the membrane surface. A disadvantage in this regard, however, is that an increased cross-flow speed is accompanied by an increased pumping requirement.
The filtrate speed is directly dependent on the pressure applied across the filter membrane, the so-called TMP (trans-membrane pressure) under the assumption of a given filter surface and otherwise equal conditions. The TMP is typically 5 bar, but there have already been isolated instances of the use of membranes working continuously at more than 10 bar.
An ideal filter membrane for the purposes of cross-flow filtration exhibits a linear flow/lifetime curve for a given fluid and otherwise unchanged conditions.
In special applications, the described cross-flow filters may also be used as so-called dead-end filters by closing the filter outlet.
Ceramic membranes for use as MF and UF membranes in cross-flow filters are already known. These typically consist of oxides of the elements aluminum, silicon, titanium and zirconium or mixtures thereof In this regard, the membranes are positioned on so-called filter carriers that are produced from the same oxides or from other ceramics, such as cordierite and silicon carbide.
The oxide ceramic membranes or so called white membranes are sintered onto a filter carrier in a large number of layers of increasingly finer grain size. The white ceramic membranes of oxide ceramic prepared in this way have a continuous and broad grain size distribution of very small grains in the micro- and sub-micro or nano-range within each layer. This also yields a very broad grain size distribution, with especially a large number of very small pores of very narrow pore channels forming that become partly sealed by the fraction of melt phase during sintering, a fact which leads to so-called dead ends, and counteracts good filtering properties on the part of the membrane. Especially, this leads to a situation in which the flow through such membranes is restricted and therefore more energy has to be expended on pumping in order that the aforementioned cross-flow rates of 2 to 8 m per second may be obtained at 2 to 5 bar TMP for MF and UF filtration.
To counteract these problems, there have already been endeavors to replace the so-called white membranes of oxide ceramic by so-called black membranes of non-oxide ceramic, such as SiC, since it is known how to make SiC carriers for filter membranes having large pores but narrow pore size distribution (see FIG. 2, left and lower section). A method for producing a corresponding filter device with an SiC filter membrane is described in WO 03/024892. In this method, primary α-SiC grains having a grain size of 1-475 μm, a silicon donor material that is not an SiC compound, organic grains having a grain size of 1×10−5 to 20 μm and at least one organic binder are molded to form a green body, then dried, and pyrolyzed in an oven under a protective atmosphere in order to transform the organic binder into a carbon binder such that the latter reacts with the melting silicon donor to form fine nanoscale β-SiC, which is then transformed at very high temperatures into very fine nanoscale α-SiC grains, which finally produce the link at the grain boundaries of the primary of α-SiC grains. This method, which involves very high outlay on account of the large number of starting materials and the high firing temperatures, also has the disadvantages that hardly any suitable silicon donor substances are available because silicon is flammable at grain sizes <6 μm and contents exceeding 100 g per m3 air must be avoided during grinding. In addition, it has already transpired during the production of filter carriers by this method that the carbon compounds formed during pyrolysis or the nano-SiC grains do not exhibit sufficient recrystallization in the usual temperature range for the recrystallization of filter carriers, with the result that, by the agency of the residual fine grains in the pore space, the strength of the ceramic body thus formed and the ability of the permeate to flow through it are impaired and no serviceable filter carriers can be produced.
The quoted range of 1 to 475 μm for the SiC primary grains is additionally much too coarse for the production of membranes for MF and UF filtration. Further, the described method of metallic silicon and carbon carriers is not suitable for the production of filter membranes because full recrystallization of the raw materials by this method requires temperatures much higher than are needed for the selective setting of uniformly fine α-SiC grains for the membranes and therefore giant grain growth in the membrane layer is unavoidable. An example of giant grain growth is shown in FIG. 6.
A further method for the production of SiC membranes is described in WO 92/11925. In this method, however, so-called binder grains are used that sinter at a temperature which is much lower than the sintering temperature of the SiC grains to be linked. This is not possible for the simple reason that pure SiC has no melting point, yet partial melting of the ceramic grains is precisely the characteristic feature of sintering. (To sinter SiC, addition of so-called sintering agents, e.g., aluminum, boron, and carbon, is necessary which generate a low quantity of melt phase. The goal here is always a dense material that can only be realized by means of shrinkage in the order of 20% and that is totally unsuitable for filter purposes).
SiC recrystallizes, however, at correspondingly high temperatures via surface diffusion or gas transport, i.e. energetically unfavorable, small grains dissolve and the material is deposited again without change in volume on energetically more favorable points, especially where two large grains make contact.
By RSiC is generally meant a recrystallized, porous material of 100 percent α-SiC as distinct from SSiC (sintered, mostly dense SiC materials with sintering agents) and oxide- or nitride-bound SiC materials or such kind with other binder phases of a different nature.
The use of chemically different binder grains in accordance with WO 92/11925, however, suffers from the disadvantage that the resistance to environmental influences and corrosion is impaired, a fact which is disadvantageous, however, for use as filter material. In addition, the chemical disparateness of filter grains and binder material also causes problems with the binding quality and the strength of the filter body. Furthermore, the use of different materials also creates difficulties during manufacture since impurities may give rise to eutectic melts that also are undesirable for the formation of a filter body. Further, by the agency of the sintering of the binder material and thus of the associated melting of this material in filter bodies, vitreous phases are present in this method that lead to sealing of the pore channels and to the formation of dead areas in the filter body, a fact which negatively affects filter performance.
In both papers on the prior art, filter membranes of SiC are apparently mentioned only as other possibilities within the context of a general listing; the key parameters that are especially important here, such as grain size distribution of the primary grain, temperature control during recrystallization and the resultant filter performance are, in contrast, not revealed.